Air conditioning size calculator
Wondering “what size air con do I need?” Enter your room and get the recommended cooling capacity in kW and BTU — using the same heat-load engine UK installers run on Vento.
A guide estimate using a Manual J-style method tuned for UK conditions. A professional survey accounts for glazing area, shading, draughts and heat gains a form can't see.
Recommended capacity
1.6–2.3 kW
≈ 6,254 BTU/hr
Get an exact quote
Have a Vento-powered installer survey the room and price the job properly.
Request a surveyHow to size air conditioning for a room
Correctly sizing an air conditioner means matching the unit's cooling capacity to the room's heat load — the heat it gains from the sun, the building fabric, the people and the equipment inside it. Get it right and the system cools quickly, runs efficiently and lasts. Get it wrong and you either never reach temperature or waste money short-cycling.
The main factors
- • Floor area & ceiling height — the volume of air to be conditioned.
- • Insulation — a solid-wall period property loses far more than a modern build.
- • Glazing & orientation — south- and west-facing glass adds significant solar gain.
- • Occupancy & equipment — people, computers and kit all add heat.
A common rule of thumb is roughly 0.10–0.15 kW per m² for a typical UK room, but that hides a lot — a sunny conservatory and a shaded north-facing bedroom of the same size need very different units. The calculator above adjusts for each factor rather than relying on a flat number.
Note: "Manual J" is the US term for this room-by-room load calculation. UK searchers usually ask "what size" or "how many BTU" — it's the same underlying method.
BTU to kW reference table
A quick guide to common unit sizes and the rooms they typically suit (1 kW ≈ 3,412 BTU). Use it as a sanity check against the calculator — your actual room may need more or less depending on glazing, orientation and insulation.
| Typical room | kW | BTU/hr |
|---|---|---|
| Small bedroom or office (up to ~15 m²) | 2.0 | ~7,000 |
| Bedroom or small lounge (~15–20 m²) | 2.5 | ~9,000 |
| Living room (~20–30 m²) | 3.5 | ~12,000 |
| Large lounge / open-plan (~30–45 m²) | 5.0 | ~17,000 |
| Large open-plan or shop (~45–60 m²) | 7.1 | ~24,000 |
| Very large space (~60–85 m²) | 10.0 | ~34,000 |
Frequently asked questions
What size air conditioner do I need for a room?
As a rough UK guide, allow around 0.10–0.15 kW of cooling per square metre of floor area for a typical, reasonably insulated room — so a 20 m² living room usually needs about 2.5–3.5 kW. Sunny, poorly insulated or south-facing rooms need more. This calculator refines that using your room dimensions, glazing, orientation and insulation.
How many BTU do I need for air conditioning?
BTU is just another way of stating capacity: 1 kW ≈ 3,412 BTU/hr. A 3.5 kW unit is roughly 12,000 BTU. Enter your room details above and the calculator shows both kW and BTU.
Is it better to oversize or undersize an air conditioner?
Neither. An undersized unit runs flat out and never cools the room; an oversized one short-cycles, wastes energy and controls humidity poorly. The aim is a capacity matched to the room's actual heat load — which is exactly what a proper survey provides.
Does this replace a professional survey?
No — it's a guide estimate. A qualified installer's survey measures glazing area, shading, air-tightness, internal heat gains and pipe routing that a quick form can't capture, and is required to specify and price the installation correctly.
Installers: size every room automatically
Vento runs a full room-by-room heat load as you survey — then designs, quotes and takes payment on the same platform. Get 35% off with code WELCOME35.
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